1. Field of the Invention
The present invention generally relates to a method of performing power control in a mobile communication system using the code division multiple access (CDMA) scheme. More particularly, it relates to a method of performing power control which is designed to adaptively control transmission power supplied from a base station to a mobile station (terminal station).
2. Description of the Related Art
It is of much importance to maximize the capacity for concurrent communications with multiple mobile stations in a frequency band supplied to a CDMA mobile communication system. A method of maximizing the capacity of a given system involves performing power control of the mobile communication system. Such methods are classified into two parts: a forward link power control for transmission power received by the terminal station from the base station, and a reverse link power control for transmission power received by the base station from the terminal station.
During the forward link power control, the mobile station located at the edge of a cell has to consume much power enough to maintain service quality to the same level as the mobile station located at a position near the base station, which is called "coner problem". Especially, the power assigned to the mobile station affects the other mobile stations as a noise in the CDMA system. Thus signal-to-noise ratio (SNR) for all mobile stations has to be kept constant by assigning less power to those mobile stations that actually need relatively small power consumption, while assigning the rest of the power to those mobile stations that require much power, in order to maximize capacity of the system.
On the contrary, the intensity of a signal received by the base station from a mobile station near the base station is stronger than that of a signal received by the base station from a mobile station located at a position far from the base station. This refers to as "near-end far-end problem". Without a proper control of transmission power, the signal output from a mobile station near the base station is received by the base station with higher intensity than is needed. The surplus power affects the signals received from the other mobile stations as a noise especially in the CDMA mobile communication system.
The forward link power control employed in the conventional CDMA mobile communication system is performed with channel elements (CEs) included in each of the mobile stations and base station (BS), and a selector provided in the base station controller (BSC). Such a conventional power control is adopted to the CDMA exchanger (product name: STAREX CMX) used in the SK Telecom.
Next, a method of performing forward control in the CDMA mobile communication system is described.
Each mobile station transmits error rate information for a forward frame applied to it. To transmit the error rate information, rate set 1 defined in the J-STD-008, PCS standard protocol with processing rate of 9.6, 4.8, 2.4 or 1.2 kbps uses a power measurement report message, while rate set 2 having processing rate of 14.4, 7.2, 3.6 or 1.8 kbps uses a one-bit error indicator bit assigned to every frames predetermined such as 20 msec or a power measurement report message. The selector increases or decreases the traffic channel gain based on the received forward frame error rate, sending the result to the channel element of the base station. Then the channel element controls the magnitude of transmission power to be supplied to the mobile station based on the traffic channel gain.
As described above, channel gain is controlled at the selector, which checks upon the forward frame error rate (FER) reported from the mobile station. If the forward FER exceeds a predetermined threshold and the current power is too weak, the selector increases the channel gain, which refers to as "upward adjustment". If the forward FER is below the predetermined threshold and the current power is strong enough, the selector decreases the channel gain, which is called "downward adjustment".
Below is a description of the upward and downward adjustments with regards to the forward link power control in a CDMA mobile communication system.
The down adjustment of the forward link power control is performed when the number of forward error frames is lower than a threshold for a given period of time. It means, the selector provided in the base station controller sets a channel gain down timer to a predetermined gain down time 101 or 301, as shown in FIGS. 1 or 3. If the forward frame error is below the threshold until the timer is terminated, the selector decrements the channel gain by a constant gain down delta 102 or 302, initializing the timer to the gain down time again. If the forward FER exceeds before the timer is terminated and the gain up adjustment is achieved, the selector initializes the timer to the gain down time again.
On the other hand, The upward adjustment of the forward link power control is performed when the number of forward error frames received from the mobile station exceeds a predetermined threshold. However, the delta of the channel gain to be adjusted upward is dependent upon the FER. That means, as shown in FIGS. 1 and 3, the gain up delta is classified into small up deltas 103 and 303, and big up deltas 104 and 304 larger than the small up deltas by a predetermined value. If the FER is below a predetermined threshold, the channel gain is increased by the small up deltas 103 and 303. If the FER exceeds the predetermined threshold, the channel gain is increased by the big up delta 104 or 304.
The conventional channel gain downward adjusting method may prevent little problems in an environment where there are little variations in the channel status because the gain down time and the gain down delta are constant. However, it is problematic in an environment where the channel status has large variations such as in the case the mobile station passes through a shadowing area, as illustrated in FIG. 1. It is thus understood that the conventional method hardly satisfy both channel statuses shown in FIGS. 1 and 3. When the channel status becomes better than in FIG. 1, or when the gain down time is set shorter than the time shown in FIG. 1 in order to reduce downward tracking time 111, a gain increase interval 311 of the gain up time in FIG. 3 becomes shorter only to increase the FER at the same power consumption.
Contrarily, in a case where the gain down time is set very long enough to be suitable for the channel status shown in FIG. 3, the downward tracking time 111 in the channel status of FIG. 1 gets prolonged to cause unnecessary power consumption, which affects as a noise the mobile stations in the cell which is adjacent to the other mobile stations therein. This causes the channel capacity of the entire communication system to be reduced. The channel environments as shown in FIGS. 1 and 3 occur very often in reality and attention must be taken to both of them in order to achieve efficient power control.
Additionally, the conventional method involves another problem that the adjustment conditions cannot be applied precisely during an upward adjustment of the channel gain. That means, the conditions of the channel gain upward adjustment are determined depending upon the result of a judgement as to whether or not the FER calculated from the number of error frames occurring in the predetermined number of frames exceeds a threshold. If the FER exceeds the threshold, the channel gain is increased by the current big up delta. If the FER is below the threshold, the channel gain is decreased by the current small up delta. However, it often happens that the channel gain increases by a big up delta even when it can be increased by a small up delta, in which case unnecessary power consumption is caused to adversely affect another mobile stations in the cell and the mobile stations in another cell. This may appear very often especially when frame errors occur continuouslly.